Neodymium salts activated with aluminum alkyl co-catalysts catalysts have been known to catalyze the polymerization of conjugated dienes since the early 1960""s. To date, many papers and patents have been published which describe variations and improvements to the original systems (see U.S. Pat. Nos. 3,297,667, 3,676,441, and 3,794,604). Much of this work was driven by the eventual commercialization of high cis-polybutadiene in the 1980s for the use in tire applications (see U.S. Pat. Nos. 4,242,232, 4,260,707, 4,699,960, and 4,444,903).
The type of catalyst system employed, and its method of preparation, are crucial to the success of the polymerization. Traditionally, there are two main types of catalyst systems, the first is a ternary system based on soluble neodymium carboxylates in conjunction with an alkylaluminum co-catalyst and a halogen source (see R. P. Quirk, A. M. Kells, K. Yunlu, J.-P. Cuif, Polymer 41, 5903 (2000) and A. Pross, P. Marquardt, K. H. Reichert, W. Nentwig, T. Knauf, Angew. Makromol. Chem. 211, 89 (1993)). The second system is a binary catalyst comprising of an insoluble neodymium halide complexed with three equivalence of a Lewis base such as an alcohol, amine, or phosphonate and an alkylaluminum activator (see H. Iovu, G. Hubca, E. Simionescu, E. Badea, J. S. Hurst, Eur. Polymer J. 33, 811 (1997); H. Iovu, G. Hubca, D. Racoti, J. S. Hurst, Eur. Polymer J. 35, 335 (1999); and J. H. Yang, M. Tsutsui, Z. Chen, D. Bergbreiter, Macromolecules 15, 230 (1982)).
In general, the two systems behave similarly; however, the ternary system appears to have gained acceptance commercially in the production of polybutadiene (see D. J. Wilson, J. Polym. Sci., Part A. 33, 2505 (1995)). Typically, the most active ternary systems consist of treating a branched long chain neodymium carboxylate with branched trialkyl-aluminum or dialkylaluminum hydrides, in an Al/Nd ratio between 10-40/1, and the use of 2-3 equivalents of a halide source, such as diethylaluminum chloride or tert-butylchloride.
The active catalyst is typically prepared in one of two ways. The simplest method is to generate the catalyst in-situ by sequentially introducing the catalyst components to the polymerization solution. It is usually most effective to introduce the aluminum alkyl components first, thereby scavenging impurities from the premix prior to contact with the neodymium salt. The other method for catalyst preparation is to preform the catalyst components prior to introducing them into the polymerization vessel. The most common practice involves sequentially treating the catalyst components in the presence of at least a few equivalents of monomer followed by an aging period. For example U.S. Pat. No. 3,794,604 discloses an improved preforming technique which is carried out in the presence of a small portion of a conjugated diene.
Aging the catalyst components with a diene prior to polymerization results in a more active catalyst than when the conjugated diene is absent. The disclosed technique for catalyst formation is to age after all of the components have been mixed together. U.S. Pat. No. 4,429,089 also teaches the use of a diolefin during catalyst formation and states that the particular procedure which is followed has no bearing on the polymerization run. Likewise, U.S. Pat. No. 4,461,883 discloses that the use of a conjugated diene in the catalyst make-up is preferable for improving the activity of the catalyst. In this example, the diene is mixed with the catalyst components at any time in the preforming step with aging occurring after all components are mixed together.
U.S. Pat. No. 4,533,711 teaches the practice of adding the catalyst components together first followed by the addition of a small amount of diene and then aging the preformed catalyst. This patent states that the diene is not essential in the make-up but it does serve to increase catalyst activity. U.S. Pat. No. 4,663,405 continues to teach the use of conjugated dienes as components in preformed catalysts. It goes on to state that soluble catalysts result when diolefins are present in the make-up while insoluble catalysts frequently result when no diene is present. This patent teaches a process where aging of the catalyst occurs after the reagents are added.
U.S. Pat. No. 5,502,126 again practices the use of a diene in the preformed catalyst make-up and again states that it is preferred to age the catalyst after the labile halogen compound is added. In U.S. Pat. No. 5,659,101 the use of a diolefin in conjunction with a boron derived halogen source results in a preformed catalyst that partially forms a solid precipitate in aliphatic solvents.
When silicone halides are used, as in U.S. Pat. No. 5,686,371, aging in the presence of a diene again is performed after the addition of all the catalyst components. U.S. Pat. No. 6,136,931 discloses an improved boron halide dependent preformed catalyst that has excellent solubility in non-polar solvents. Finally, U.S. Pat. No. 6,255,416 also practice preformed catalyst generation in the presence of a small amount of diene. Aging in this case again occurs after all of the catalyst components are mixed.
The technique of combining a neodymium salt, an aluminum alkyl, a halide source, and a diene to attain an improved result is the subject of this invention. As the prior art describes, almost any conjugated diene monomer can be used in the preforming step, and that each diene can be treated in the same way. For example, prior teachings imply that the contact time between the conjugated diene and the neodymium/aluminum co-catalyst treatment step is not crucial and that aging should occur after the halide source has been added. However, we have now found that a minimum contact time does indeed exist for different conjugated dienes when the preparation of a completely soluble catalyst is desired. It is also crucial that this contact time occurs prior to the introduction of a halide source in order to ensure completely soluble catalyst solutions. For example, formation of a homogeneous catalyst solution is achieved when isoprene is used in the preform only if the isoprene/neodymium/aluminum alkyl solution is allowed to age for an extended amount of time prior to aluminium-chloride addition. If the first step is not allowed to proceed long enough, a precipitate is formed upon addition of aluminum-chloride. When butadiene is used in the preforming reaction this first aging period is still crucial, yet, significantly less time is needed to ensure a homogenous catalyst.
The technological advantage of a completely soluble preformed catalyst has previously been appreciated. As U.S. Pat. No. 4,461,883 teaches, a heterogeneous system is a disadvantage in an industrial setting. Likewise, U.S. Pat. No. 6,136,931 states that the use of heterogeneous catalyst systems containing suspended particles usually produces gel. This patent also states a heterogeneous system, compared to a homogenous one, is more difficult to control the exact amount of catalyst added during the polymerization. Similarly, we have found that catalyst prepared without the first aging period results in a catalyst suspension of a fine precipitate. This suspension settles upon standing into two phases. If the resulting supernate, or top layer, is used to polymerize a conjugated diene, extremely inefficient catalyst activity results. Catalyst activity can be restored in these systems only after agitation of the by-phasic mixtures. This allows for the introduction of a heterogeneous catalyst suspension to the monomer to be polymerized. However, it is now possible to ensure consistent and highly active soluble preformed catalyst formation by utilizing the appropriate two step aging technique. This is of obvious technological advantage, since there would be no need to use a stirred tank catalyst storage tank or other engineering constraints to ensure consistent catalyst suspensions.
The neodymium catalyst system prepared by the technique of this invention can be used in the polymerization of isoprene monomer into polyisoprene rubber that is clear (transparent) and of high purity. This invention more specifically discloses a process for the synthesis of polyisoprene rubber which comprises polymerizing isoprene monomer in the presence of a neodymium catalyst system, wherein the neodymium catalyst system is prepared by (1) reacting a neodymium carboxylate with an organoaluminum compound in the presence of isoprene for a period of about 10 minutes to about 30 minutes to produce neodymium-aluminum catalyst component, and (2) subsequently reacting the neodymium-aluminum catalyst component with a dialkyl aluminum chloride for a period of at least 30 minutes to produce the neodymium catalyst system.
The neodymium catalyst system of this invention can be used in the polymerization of isoprene monomer into polyisoprene rubber that is clear and of high purity. Such polymerizations are typically conducted in a hydrocarbon solvent that can be one or more aromatic, paraffinic, or cycloparaffinic compounds. These solvents will normally contain from 4 to 10 carbon atoms per molecule and will be liquids under the conditions of the polymerization. Some representative examples of suitable organic solvents include pentane, isooctane, cyclohexane, normal hexane, benzene, toluene, xylene, ethylbenzene, and the like, alone or in admixture.
In solution polymerizations that utilize the catalyst systems of this invention, there will normally be from 5 to 35 weight percent isoprene monomer in the polymerization medium. Such polymerization mediums are, of course, comprised of an organic solvent, the isoprene monomer, and the catalyst system. In most cases, it will be preferred for the polymerization medium to contain from 10 to 30 weight percent isoprene monomer. It is generally more preferred for the polymerization medium to contain 12 to 18 weight percent isoprene monomer.
The neodymium catalyst system used in the process of this invention is made by preforming three catalyst components. These components are (1) an organoaluminum compound, (2) a neodymium carboxylate, and (3) a dialkyl aluminum chloride. In making the neodymium catalyst system the neodymium carboxylate and the organoaluminum compound are first reacted together for 10 minutes to 30 minutes in the presence of isoprene to produce a neodymium-aluminum catalyst component. The neodymium carboxylate and the organoaluminum compound are preferable reacted for 15 minutes to 25 minutes.
The neodymium-aluminum catalyst component is then reacted with the dialkyl aluminum chloride for a period of at least 30 minutes to produce the neodymium catalyst system. The activity of the neodymium catalyst system normally improves as the time allowed for this step is increased up to about 24 hours. Greater catalyst activity is not normally attained by increasing the aging time over 24 hours. However, the catalyst system can be aged for much longer time periods before being used with out any detrimental results.
The neodymium catalyst system will typically be preformed at a temperature that is within the range of about xe2x88x9210xc2x0 C. to about 60xc2x0 C. The neodymium catalyst system will more typically be prepared at a temperature that is within the range of about 0xc2x0 C. to about 30xc2x0 C. The neodymium catalyst system will preferably be prepared at a temperature that is within the range of about 5xc2x0 C. to about 15xc2x0 C. The period of time needed for the catalyst to be preformed is increased by utilizing lower temperatures.
In a highly preferred embodiment of this invention the dialkyl aluminum chloride is slowly added to the neodymium-aluminum catalyst component over a period of at least 30 minutes. The dialkyl aluminum chloride is preferable slowly added to the neodymuim-aluminum catalyst component over a period of at least 45 minutes and is most preferable added over a period of at least 60 minutes. By slowly adding the dialkyl aluminum chloride to the neodymium-aluminum catalyst component over an extended period of time the first step of the process wherein the neodymium carboxylate is reacted with the organoaluminum compound is much more forgiving. More specifically, the reaction time need for the first step is not as critical and can be reduced to as short of a period as 1 minute or extended to a period as long as 12 hours at low temperatures. In this embodiment of the subject invention the neodymium carboxylate will preferable be allowed to react with the organoaluminum compound in the presence of isoprene for a period of 2 hours to 8 hours at a temperature which is within the range of 0xc2x0 C. to 30xc2x0 C. In this embodiment of the subject invention the neodymium carboxylate will more preferable be allowed to react with the organoaluminum compound in the presence of isoprene for a period of 4 hours to 6 hours at a temperature which is within the range of 5xc2x0 C. to 15xc2x0 C.
The organoaluminum compound contains at least one carbon to aluminum bond and can be represented by the structural formula: 
in which R1 is selected from the group consisting of alkyl (including cycloalkyl), alkoxy, aryl, alkaryl, arylalkyl radicals and hydrogen: R2 is selected from the group consisting of alkyl (including cycloalkyl), aryl, alkaryl, arylalkyl radicals and hydrogen and R3 is selected from a group consisting of alkyl (including cycloalkyl), aryl, alkaryl and arylalkyl radicals. Representative of the compounds corresponding to this definition are: diethylaluminum hydride, di-n-propylaluminum hydride, di-n-butylaluminum hydride, diisobutylaluminum hydride, diphenylaluminum hydride, di-p-tolylaluminum hydride, dibenzylaluminum hydride, phenylethylaluminum hydride, phenyl-n-propylaluminum hydride, p-tolylethylaluminum hydride, p-tolyl-n-propylaluminum hydride, p-tolylisopropylaluminum hydride, benzylethylaluminum hydride, benzyl-n-propylaluminum hydride, and benzylisopropylaluminum hydride and other organoaluminum hydrides. Also included are ethylaluminum dihydride, butylaluminum dihydride, isobutylaluminum dihydride, octylaluminum dihydride, amylaluminum dihydride and other organoaluminum dihydrides. Also included are diethylaluminum ethoxide and dipropylaluminum ethoxide. Also included are trimethylaluminum, triethylaluminum, tri-n-propylaluminum, triisopropylaluminum, tri-n-propylaluminum, triisopropylaluminim, tri-n-butylaluminum, triisobutylaluminum, tripentylaluminum, trihexylaluminum, tricyclohexylaluminum, trioctylaluminum, triphenylaluminum, tri-p-tolylaluminum, tribenzylaluminum, ethyldiphenylaluminum, ethyl-di-p-tolylaluminum, ethyldibenzylaluminum, diethylphenylaluminum, diethyl-p-tolylaluminum, diethylbenzylaluminum and other triorganoaluminum compounds.
The neodymium carboxylate utilizes an organic monocarboxylic acid ligand that contains from 1 to 20 carbon atoms, such as acetic acid, propionic acid, valeric acid, hexanoic acid, 2-ethylhexanoic acid, neodecanoic acid, lauric acid, stearic acid and the like neodymium naphthenate, neodymium neodecanoate, neodymium octanoate, and other neodymium metal complexes with carboxylic acid containing ligands containing from 1 to 20 carbon atoms.
The proportions of the catalyst components utilized in making the neodymium catalyst system of this invention can be varied widely. The atomic ratio of the halide ion to the neodymium metal can vary from about 0.1/1 to about 6/1. A more preferred ratio is from about 0.5/1 to about 3.5/1 and the most preferred ratio is about 2/1. The molar ratio of the trialkylaluminum or alkylaluminum hydride to neodymium metal can range from about 4/1 to about 200/1 with the most preferred range being from about 8/1 to about 100/1. The molar ratio of isoprene to neodymium metal can range from about 0.2/1 to 3000/1 with the most preferred range being from about 5/1 to about 500/1.
The amount of catalyst used to initiate the polymerization can be varied over a wide range. Low concentrations of the catalyst system are normally desirable in order to minimize ash problems. It has been found that polymerizations will occur when the catalyst level of the neodymium metal varies between 0.05 and 1.0 millimole of neodymium metal per 100 grams of monomer. A preferred ratio is between 0.1 and 0.3 millimole of neodymium metal per 100 grams of monomer.
The concentration of the total catalyst system employed of course, depends upon factors such as purity of the system, polymerization rate desired, temperature and other factors. Therefore, specific concentrations cannot be set forth except to say that catalytic amounts are used.
Temperatures at which the polymerization reaction is carried out can be varied over a wide range. Usually the temperature can be varied from extremely low temperatures such as xe2x88x9260xc2x0 C. up to high temperatures, such as 150xc2x0 C. or higher. Thus, the temperature is not a critical factor of the invention. It is generally preferred, however, to conduct the reaction at a temperature in the range of from about 10xc2x0 C. to about 90xc2x0 C. The pressure at which the polymerization is carried out can also be varied over a wide range. The reaction can be conducted at atmospheric pressure or, if desired, it can be carried out at sub-atmospheric or super-atmospheric pressure. Generally, a satisfactory polymerization is obtained when the reaction is carried out at about autogenous pressure, developed by the reactants under the operating conditions used.
The polymerization can be terminated by the addition of an alcohol or another protic source, such as water. Such a termination step results in the formation of a protic acid. However, it has been unexpectedly found that better color can be attained by utilizing an alkaline aqueous neutralizer solution to terminate the polymerization. Another advantage of using an alkaline aqueous neutralizer solution to terminate the polymerization is that no residual organic materials are added to the polymeric product.
Polymerization can be terminated by simply adding an alkaline aqueous neutralizer solution to the polymer cement. The amount of alkaline aqueous neutralizer solution added will typically be within the range of about 1 weight percent to about 50 weight percent based upon the weight of the polyisoprene cement. More typically, the amount of the alkaline aqueous neutralizer solution added will be within the range of about 4 weight percent to about 35 weight percent based upon the weight of the polyisoprene cement. Preferable, the amount of the alkaline aqueous neutralizer solution added will be within the range of about 5 weight percent to about 15 weight percent based upon the weight of the polyisoprene cement.
The alkaline aqueous neutralizer solution will typically have a pH which is within the range of 7.1 to 9.5. The alkaline aqueous neutralizer solution will more typically have a pH which is within the range of 7.5 to 9.0, and will preferable have a pH that is within the range of 8.0 to 8.5. The alkaline aqueous neutralizer solution will generally be a solution of an inorganic base, such as a sodium carbonate, a potassium carbonate, a sodium bicarbonate, a potassium bicarbonate, a sodium phosphate, a potassium phosphate, and the like. For instance, the alkaline aqueous neutralizer solution can be a 0.25 weight percent solution of sodium bicarbonate in water. Since the alkaline aqueous neutralizer solution is not soluble with the polymer cement it is important to utilize a significant level of agitation to mix the alkaline aqueous neutralizer solution into throughout the polymer cement to terminate the polymerization. Since the alkaline aqueous neutralizer solution is not soluble in the polymer cement it will readily separate after agitation is discontinued.
This invention is illustrated by the following examples which are merely for the purpose of illustration and are not to be regarded as limiting the scope of the invention or the manner in which it can be practiced. Unless specifically indicated otherwise, parts and percentages are given by weight.